1. Field of the Invention
This invention relates to a double cylinder body and a manufacturing method thereof, more particularly, to an inside and outside cylindrical body each of which may define differing inner and outer diameters, and wherein the inside cylindrical body is connected to an inner circumference defined by the outside cylindrical body. The present invention can be effectively employed, for example, in a housing of a fan motor mounted on a vehicle and the like.
2. Description of the Related Art
In general, a housing of a fan motor mounted on an vehicle is required to be as light as possible from the point of view of improving fuel consumption and the like. A fan motor used on an engine radiator is required high to have radiating property.
A fan motor complying with the requirement is therefore proposed in the Japanese laying open of application No. 4-347548. In this fan motor, an iron-made yoke is connected to the inside of an aluminum-made housing, making it lightweight and the radiating property thereof is improved by using the aluminum-made casing.
When the iron-made yoke is connected to the inside of the aluminum-made housing, the connecting method is generally brazing, a rivet or the like is generally adopted.
However, in the case in which the housing is manufactured by brazing, a rivet or the like, the problems often occur.
(1) It is difficult to judge the connection between the yoke and the housing by a nondestructive inspection. PA1 (2) The connecting step(s) step is(are) necessary as a special step, so that quantity production is prevented. PA1 (3) A medium is necessary except for the connecting parts. PA1 (a) It is difficult to verify all connections. PA1 (b) It is difficult to determine the connecting strength. PA1 (c) It is difficult to simplify the connecting steps.
It has been considered to manufacture the housing by a connecting method such as caulking, or metal flow. However, the above problems (1) and the (2) remain in the connecting method of caulking, and the predetermination of the connecting strength is difficult. The above problems (1), (2) and (3) remain in a connecting method by metal flow.
That is to say, the problems as follows exist even using the above connecting method.